SERS and DFT Investigations of Methyl 4-bromo-1H-pyrrole-2-carboxylate adsorbed on Silver and Gold Substrates: In Perspective of Biosensor Applications

Abstract In the present study, the spontaneous Raman spectrum and surface-enhanced Raman scattering (SERS) spectra of Methyl 4-bromo-1h-pyrrole-2-carboxylate (MBPC) were reported and validated using density functional theory (DFT) quantum chemical calculations. Initially, the molecular structure of MBPC, MBPC-Ag3 and MBPC-Au4 molecular systems were optimized. In order to attain the strong SERS enhancement, the SERS substrates such as silver-coated thin film (SCF) and gold-coated thin film (GCF) were fabricated. SEM images of SCF and GCF show that the silver and gold nanoparticles are spherical in shape with an average particle size of ∼74 and ∼56 nm, respectively. EDX spectra of SCF and GCF clearly evidences that only the silver and gold nanoparticles present in the respective films. The UV-visible spectral analysis shows a significant red-shift upon the MBPC molecule adsorbed on SCF and GCF surfaces, confirming that the MBPC molecule chemisorbed on SCF and GCF surfaces. Frontier molecular orbitals analysis shows that the charge transfer interaction occurs between the metal and molecule in the MBPC-Ag3 and MBPC-Au4 molecular systems. The molecular electrostatic potential surface and Fukui functions analysis further validated the charge transfer interaction in the investigated molecular systems. The spontaneous Raman and SERS spectral analysis strongly evidence that the title molecule chemisorbed on GCF with high affinity than SCF with tilted orientation through the lone pair electrons of the oxygen atom in the C=O functional group, pyrrole ring π electrons and lone pair electrons of bromine atom. Moroever, the experimentally observed results were correlated well with theoretical results. Hence, the present study paves the way for designing strong SERS active substrates, which will be useful for developing pyrrole related biosensors.